isChordal -- determines if a graph is chordal

A graph is chordal if the graph has no induced cycles of length 4 or more (triangles are allowed). To check if a graph is chordal, we use a characterization of Fr\"oberg (see "On Stanley-Reisner rings," Topics in algebra, Part 2 (Warsaw, 1988), 57-70, Banach Center Publ., 26, Part 2, PWN, Warsaw, 1990.) that says that a graph G is chordal if and only if the edge ideal of G^c has a linear resolution, where G^c is the complementary graph of G.